The general objective is to increase our understanding of the biology of carcinogenesis. Carcinogenesis is interpreted to mean the whole series of events and processes that result in a fully developed, metastasizing neoplasm. A major portion of the work is therefore concerned with the interactions of a neoplastically transformed cell or clone with the host. In particular, emphasis will be given to the investigation of a newly discovered property of the immune response, namely specific tumor immunological stimulation. For this purpose both in vivo transplantation studies and in vitro cytotoxicity tests will be used. The "nude" mouse in the germ-free state will be a principal tool. Additionally, the mechanisms of the immunological stimulation will be investigated with particular reference to T-cell and B-cell function and the possible role of specifically immune serum. It is hoped that these studies will give insight into ways in which immunological stimulation can be prevented and immunological surveillance made more effective. Bibliographic references: Outzen, H. C., Ross, S. E., and Prehn, R. T. Cell-mediated immunologic responses of germfree "nude" mice. In: Proc. 1st Int. Workshop on Nude Mice, 1974, J. Rygaard and C. O. Povlsen, Eds., Gustav Fischer Verlag, Stuttgart, 1974, pp. 209-214. Prehn, R. T. Does immunity promote or inhibit tumor growth? In: Viruses and Immunity, Academic Press, New York, 1975, pp. 59-66.